The arrangement of phosphate groups in the phospholipid bilayer of the plasma membrane is due to their hydrophilic (water-loving) nature. Phosphate groups contain a negative charge, making them polar and attracted to water molecules. As a result, they face towards the outside of the membrane, which is in contact with the extracellular fluid or the aqueous environment.
On the other hand, fatty acids, which are hydrophobic (water-repelling), face inward, away from the watery environment. This arrangement allows the hydrophobic tails of the phospholipids to interact with each other, creating a stable lipid bilayer structure. This lipid bilayer acts as a barrier, preventing the free movement of water-soluble molecules across the membrane while allowing the passage of lipid-soluble substances.